Synastry (星盤合盤)

Relationship Astrology — the Aspects Between Two Charts

Overview

Synastry is the branch of Western astrology that compares two birth charts to reveal the chemistry between them. By measuring the angles (aspects) each person's planets make to the other's — conjunctions, sextiles, squares, trines, and oppositions — it maps where two people flow together and where they generate friction. Sun-Moon, Venus-Mars, and Sun-Venus contacts are the classic signatures of attraction and ease; hard aspects show the growth edges of a bond.

Origin & history

Synastry grows out of the Hellenistic and medieval astrological tradition of chart comparison, formalised over centuries as the doctrine of aspects matured. The modern practice — overlaying two natal charts and reading the inter-aspects — became a staple of 20th-century Western astrology and relationship counselling.

As Western astrology professionalised through the 20th century, synastry became one of its most-requested services: couples, families, and partners seeking to understand their dynamics. The rise of astrology software made precise inter-aspect grids instant, and synastry is now among the most popular applications of the natal chart.

How it works

Each person's natal positions for the ten planets (Sun through Pluto) are computed from birth data. The engine then measures the angular separation between every planet of chart A and every planet of chart B; where that separation falls within orb of a major aspect, an inter-aspect is recorded. Harmonious aspects (trine, sextile, and soft conjunctions) and challenging aspects (square, opposition, and hard conjunctions) are tallied — weighted toward the personal planets — into a 0–100 compatibility score, with the key Sun/Moon and Venus/Mars contacts surfaced.

Good for

• Romantic compatibility
• Understanding relationship dynamics
• Friendship and family chemistry
• Business-partner fit
• Seeing both the ease and the growth edges
• A Western complement to BaZi or Kundli matching

Use cases

Compatibility Snapshot

Enter two birth dates and receive the inter-aspect grid, a harmonious-vs-challenging tally, the key Sun/Moon/Venus/Mars contacts, and a 0–100 compatibility score with a verdict.

Relationship Counselling

Hard aspects (squares, oppositions) pinpoint where two people rub — the conscious work that, done well, deepens a relationship rather than ending it.

Beyond Romance

Synastry applies equally to friends, family, and co-founders; the planets involved show which sphere — affection, drive, communication, responsibility — each contact governs.

Cross-Tradition Matching

Pair Western synastry with Chinese BaZi compatibility or Vedic Kundli Milan for a multi-system relationship reading.

Key terms

Aspect

An angle between two planets — conjunction (0°), sextile (60°), square (90°), trine (120°), opposition (180°) — within a tolerance called the orb.

Inter-aspect

An aspect between a planet in one chart and a planet in the other — the unit of synastry.

Orb

The allowable distance from an exact aspect; tighter orbs indicate stronger contacts.

Personal planets

Sun, Moon, Mercury, Venus, and Mars — the fast-moving bodies whose contacts carry the most weight in relationship chemistry.

API

The askTIAN Synastry API computes every inter-aspect between two natal charts on a verified pure-JS ephemeris, returning the key Sun/Moon/Venus/Mars contacts, a harmonious-vs-challenging tally, the full aspect list, and a 0–100 compatibility score. Add birth times for accurate Moon and Ascendant aspects.

Endpoint: POST /trpc/forecast.synastry — 3 TIAN Points. See the API documentation and Playground.